1. Field of the Invention
The present invention relates to a method and an apparatus for selecting a public land mobile network (PLMN). More specifically, this invention relates to a method and an apparatus that can shorten the time needed to select a PLMN.
2. Description of the Prior Art
When a user turns on a mobile phone, the mobile phone must select a PLMN for providing communication services. According to the Global System for Mobile communication (GSM) specifications, when a mobile phone selects a PLMN, the registered public land mobile network (RPLMN) that the mobile phone registered last time is the first priority PLMN, and the home public land mobile network (HPLMN) for the mobile phone is the second priority PLMN. In other words, when selecting a PLMN, a mobile phone must select a PLMN with higher priority according to the specifications. Only when the first priority PLMN (for example, RPLMN) cannot be selected, a mobile phone can select the second priority PLMN (for example, HPLMN) to provide communication services.
Every mobile phone has a base station table. The first priority PLMN cannot be selected when there is no base station in the base station table belonging to the first priority PLMN. Because no base station belonging to the first priority PLMN can be selected, the mobile phone cannot select the first priority PLMN.
Please refer to FIG. 1. FIG. 1 shows a base station table for a prior art method of selecting a PLMN. When a mobile phone is turned on, the mobile phone monitors the broadcasting information broadcasted by a plurality of base stations in the vicinity. Assume the mobile phone can monitor broadcasting information broadcasted by ten base stations. After receiving and measuring the broadcasting information of the ten base stations, the mobile phone establishes a base station table as shown in FIG. 1. The base station table includes a base station code field (cell_arfcn) for recording the base station codes of the ten base stations and a base station power field (cell_rlac) for recording the power values of the ten pieces of broadcasting information broadcasted by the ten base station. As shown in FIG. 1, the power values of the ten pieces of broadcasting information are recorded in the base station table in sequence according to the magnitudes of the power values (the upper ones have larger power values than the lower ones).
In the prior art method for selecting a PLMN, a mobile phone sequentially searches the PLMN to which each of the base stations in the base station table belongs. In other words, the mobile phone first searches the PLMN to which the base station 10 having a base station code “10” belongs.
Before reading the broadcasting information broadcasted by a base station, a mobile station, such as a mobile phone, must first synchronize with the base station. According to the third system information (SI3) included in the broadcasting information, the PLMN to which the base station belongs can be known. Therefore, the mobile station can determine whether a base station belongs to the first priority PLMN according to the SI3 for the base station. If the determined result is YES, then the mobile station selects the base station as a serving base station for providing communication services, i.e. the mobile station selects the PLMN to which the base station belongs as the serving PLMN for providing communication services.
As mentioned above, a mobile phone sequentially searches the PLMN to which each of the base stations in the base station table belongs. When the searched PLMN to which a base station belongs is the first priority PLMN, the mobile station selects the base station as a serving base station.
If the base station corresponding to the greatest power value of the broadcasting information in the base station table does not belong to the first priority PLMN, the mobile station repeats the foregoing steps for the next base station in the base station table until a base station belonging to the first priority PLMN is found. If no base station belonging to the first priority PLMN is found after all the base stations in the base station table are searched, the mobile station sets the second priority PLMN as the next target PLMN. Foregoing steps are repeated for searching a base station belonging to the second priority PLMN.
Accordingly, the prior art method for selecting a PLMN spends much time on sequentially searching the PLMN for each of the base stations in the base station table and determining whether the searched result conforms to the target PLMN, for instance, the first priority PLMN, the second priority PLMN, etc. Besides, a mobile station according to prior art must first synchronize with each of the base stations before determining whether the base station belongs to the target PLMN according to the third system information of the base station. Thus, if the base stations corresponding to the larger power values of broadcasting information in the base station table do not belong to the first priority PLMN, the prior art method for selecting a PLMN has to spend much time on sequentially synchronizing with these base stations and reading their third system information.
Furthermore, when a user uses his mobile phone in a foreign country, a roaming PLMN is selected because the home public land mobile network (HPLMN) cannot be selected in the foreign country. After the user goes back to his home country and turns on the mobile phone for the first time, the registered public land mobile network (RPLMN) for the mobile phone is still the roaming PLMN, and because the mobile phone set its RPLMN as the first priority PLMN, the mobile phone first searches the base station belonging to the roaming PLMN. However, there is no base station belonging to the roaming PLMN in the base station table. Even though the mobile phone cannot find any base station belonging to the RPLMN, the mobile station still takes time in synchronizing with all the base stations in the base station table and in reading their third system information. Only after confirming that there's no base station belonging to the RPLMN, the mobile phone starts to search a base station belonging to the HPLMN. Under foregoing conditions, the prior art method for selecting a PLMN is very ineffective.